1. Field of the Invention
The present invention relates to an optical fiber amplifier based on a wavelength division multiplexing system. More particularly, the present invention relates to an automatic gain controller of an optical fiber amplifier for making a gain per channel constant when the number of input channels is varied.
2. Description of the Related Art
In an optical transmission system, the rearrangement of networks, the adding/dropping of signal channels, as well as the failure of transmission lines and so forth are responsible for a change in the number of multiplexed channels available at a given instant of time. A change in the number of input channels in an optical fiber amplifier generated by any of the above-mentioned reasons (i.e., a change in input optical power) causes a reverse rate of density (population inversion) of an optical fiber, such as an erbium-doped optical fiber, as a determining factor of the momentary change in gain. In addition, the channels that survive in the optical fiber are subjected to a transient excursion of output power. This transient excursion of the output power is brought about by a nonlinear phenomenon in the optical fiber, or by reducing a signal-to-noise ratio to restrict a performance of the transmission system. In order to solve this problem, it has been proposed an automatic gain control optical fiber amplifier having a constant gain at all times regardless of a variable amount of input channels connected at a given instant of time. A considerable number of optical fiber amplifier products, to which an automatic gain control mode is provided as an additional function, mainly employ a mode in which a bias current of a pump laser diode (LD) is electrically controlled. The reason for this is because when compared with the automatic gain control optical fiber amplifier that is optically implemented, it is convenient to construct them, and that it is easy to implement them through a simple control circuit.
The mode in which the bias current of the pump LD in the optical fiber amplifier is controlled to implement the automatic gain control is generally exemplified by three modes, (1st) a feed forward mode, (2md) a feedback mode and a (3d) mode using a micro controller.
The feed forward mode performs automatic gain control in a manner in which a current is converted into a voltage, and then the voltage is directly applied to a bias current driver of a pump LD, wherein the current is then input through a pin photodiode (PIN-PD) connected on one side of an optical tap coupler located on an input side of an optical fiber amplifier. The feed forward mode has an advantage in that it has a rapid control speed and a simple circuit construction, but a disadvantage in that it is sensitive to a change in characteristics of optical/electronic devices used in the optical fiber amplifier, so that it is difficult to perform precise automatic gain control.
To secure a stability compared with the feed forward mode, the feedback mode is presently used in most of the automatic gain control optical fiber amplifiers.
FIG. 1 shows a construction of a conventional optical fiber amplifier using a feedback mode.
Referring to FIG. 1, the feedback mode causes a current input through a PIN-PD 3 connected on one side of an optical tap coupler 2 located on an output side of an optical fiber amplifier to be converted into a voltage (TIA2; 4), and then feeds the converted voltage back to a “+” (i.e. non-inverting) terminal of a differential amplifier 5. Normally, a value of the converted voltage refers to a present value or a real value. There is a comparison of a reference value (i.e., a value of a voltage proportional to the number of the channels into which a voltage converted by a PIN-PD on an input side of an optical fiber amplifier is applied) input into a “−” (i.e. inverting) terminal of the differential amplifier with a fed back present value, PI (Proportional & Integral) controller 6 is operated until these two values become equal to each other, and controls a pump LD driver 8 which outputs a bias current for driving a pump LD 7.
In addition, with regard to FIG. 1, reference numerals, 2, 3 and 4 refer to an optical tap coupler located on the input side of the optical fiber amplifier, the PIN-PD and a TIA1 (Trans Impedance Amplifier 1) respectively, 9 refers to an adder, 10 is a transient suppressor for suppressing a transient phenomenon, and 11 is a gain amplifier.
However, as shown in FIGS. 2 and 3, PI control of the conventional feedback mode is characterized in that, as a variation of the number of input channels active on the input side of the optical fiber amplifier is increased, both a transient power excursion and a transient response time are deteriorated. In other words, as a variation of input optical power increases, both the transient power excursion and transient response time is increased. Accordingly, a time constant allowing for optimization of the transient phenomenon must be set whenever conditions of input channels are changed. For this reason, the PI control requires a separate suppressing method.
FIG. 2 is a graph showing a variation of optical power of survival channels in an optical fiber according to a change of the channels when a time constant is 10 μs, and FIG. 3 is a graph showing a variation of optical power of survival channels in an optical fiber according to a change of the channels when a time constant is 74 μs. When the time constant is set from 10 μs (FIG. 2) to 74 μs (FIG. 3), while it can be seen that a phenomenon of the transient power excursion A is ameliorated to a certain extent, but the transient response time B is actually lengthened.
Meanwhile, the micro controller mode performs control in a manner that carries out a PI operation in a micro controller, and then transmits the operated result to a bias current driver through a digital/analog (D/A) converter. However, the conventional commercialized micro controller mode has an internal operational processing speed that is an important parameter thereby limiting speed control of the transient phenomenon of the optical amplifier by means of the micro controller.